Functions

A function is a named, reusable block of code that performs a specific task. Functions help you avoid repetition, organise your code, and make it easier to test and maintain.

Defining a Function

Use the def keyword:

def greet(name):
    message = f"Hello, {name}!"
    return message

result = greet("Alice")
print(result)   # Hello, Alice!

The function body is indented. return sends a value back to the caller. Calling the function by name with parentheses executes it.

Parameters and Arguments

Parameters are variables listed in the function definition. Arguments are the actual values passed when calling the function:

def add(a, b):
    return a + b

print(add(3, 4))    # 7
print(add(10, -2))  # 8

Default Parameter Values

You can provide defaults so parameters become optional:

def greet(name, greeting="Hello"):
    return f"{greeting}, {name}!"

print(greet("Alice"))              # Hello, Alice!
print(greet("Bob", "Good morning")) # Good morning, Bob!

Keyword Arguments

Call functions with argument names for clarity:

def describe_pet(name, species):
    print(f"{name} is a {species}")

describe_pet(species="cat", name="Whiskers")

Return Values

A function can return any value — or nothing (implicitly returns None):

def square(x):
    return x * x

def say_hello():
    print("Hello!")   # returns None

result = square(5)   # 25

You can return multiple values as a tuple:

def min_max(numbers):
    return min(numbers), max(numbers)

low, high = min_max([3, 1, 7, 2, 9])
print(low, high)   # 1 9

Variable Scope

Variables created inside a function are local — they only exist within that function:

def my_function():
    local_var = 10
    print(local_var)   # 10

my_function()
# print(local_var)   # NameError — not accessible here

Variables defined outside functions are global:

PI = 3.14159

def circle_area(radius):
    return PI * radius ** 2

print(circle_area(5))   # 78.53975

Docstrings

Document your functions with a docstring — a string literal on the first line of the body:

def celsius_to_fahrenheit(c):
    """Convert Celsius to Fahrenheit and return the result."""
    return c * 9 / 5 + 32

help(celsius_to_fahrenheit)

*args and **kwargs

For variable numbers of arguments:

def total(*args):
    return sum(args)

print(total(1, 2, 3, 4))   # 10

def show_info(**kwargs):
    for key, value in kwargs.items():
        print(f"{key}: {value}")

show_info(name="Alice", age=30)

Functions are the building blocks of well-structured programs. Well-named, focused functions make code dramatically easier to read and debug.